Photopolymer composition

ABSTRACT

The present disclosure is to provide a photopolymer composition including a polymer matrix or a precursor thereof having a specific chemical structure; a photoreactive monomer; and a photoinitiator. The present disclosure is also to provide a hologram recording medium, an optical element, and a holographic recording method using the photopolymer composition.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a 35 U.S.C. 371 National Phase Entry Application from PCT/KR2018/015466, filed Dec. 7, 2018, which claims the benefits of Korean Patent Applications No. 10-2017-0169489 filed on Dec. 11, 2017 and No. 10-2018-0156150 filed on Dec. 6, 2018 with the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference in entirety.

TECHNICAL FIELD

The present disclosure relates to a photopolymer composition, a hologram recording medium, an optical element and a holographic recording method.

BACKGROUND OF ART

Hologram recording medium records information by changing a refractive index in the holographic recording layer in the medium through an exposure process, reads the variation of refractive index in the medium thus recorded, and reproduces the information.

When a photopolymer (photosensitive resin) is used, the light interference pattern can be easily stored as a hologram by photopolymerization of the low molecular weight monomer. Therefore, the photopolymer can be used in various fields such as optical lenses, mirrors, deflecting mirrors, filters, diffusing screens, diffraction elements, light guides, waveguides, holographic optical elements having projection screen and/or mask functions, medium of optical memory system and light diffusion plate, optical wavelength multiplexers, reflection type, transmission type color filters, and the like.

Typically, a photopolymer composition for hologram production comprises a polymer binder, a monomer, and a photoinitiator, and the photosensitive film produced from such a composition is irradiated with laser interference light to induce photopolymerization of local monomers.

In a portion where a relatively large number of monomers are present in such photopolymerization process, the refractive index becomes high. And in a portion where a relatively large number of polymer binders are present, the refractive index is relatively lowered and thus the refractive index modulation occurs, and a diffraction grating is generated by such refractive index modulation. The refractive index modulation value (n) is influenced by the thickness and the diffraction efficiency (DE) of the photopolymer layer, and the anglular selectivity increases as the thickness decreases.

Recently, development of materials capable of maintaining a stable hologram with a high diffraction efficiency has been demanded, and also various attempts have been made to manufacture a photopolymer layer having a thin thickness and a high refractive index modulation value.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

The present disclosure is to provide a photopolymer composition which can more effectively and easily provide a photopolymer layer having a high refractive index modulation value even with a thin thickness.

The present disclosure is also to provide a hologram recording medium including a photopolymer layer having a high refractive index modulation value even with a thin thickness.

The present disclosure is also to provide an optical element including the above-described hologram recording medium.

The present disclosure is also to provide a holographic recording method including selectively polymerizing photoreactive monomers contained in the photopolymer composition using a coherent laser.

Technical Solution

The present disclosure provides a photopolymer composition, including a polymer matrix or a precursor thereof including a (meth)acrylate-based (co)polymer having a silane-based functional group in a branched chain and a silane cross-linking agent; a photoreactive monomer; and a photoinitiator, wherein a ratio of siloxane functional groups represented by the following Chemical Formula 1 to total siloxane functional groups in the silane cross-linking agent is 90 mol % or more according to a solid state ²⁹Si NMR analysis of the silane cross-linking agent:

in Chemical Formula 1, R₁ to R₃ are each an organic functional group.

The present disclosure also provides a hologram recording medium produced from the photopolymer composition.

The present disclosure also provides an optical element including the hologram recording medium.

The present disclosure also provides a holographic recording method including selectively polymerizing photoreactive monomers contained in the photopolymer composition using a coherent laser.

Hereinafter, the photopolymer composition, the hologram recording medium, the optical element, and the holographic recording method according to a specific embodiment of the present invention will be described in more detail.

As used herein, the term “(meth)acrylate” refers to either methacrylate or acrylate.

As used herein, the term “(co)polymer” refers to either a homopolymer or copolymer (including random copolymers, block copolymers, and graft copolymers).

Further, the term “hologram” as used herein refers to a recording medium in which optical information is recorded in an entire visible range and a near ultraviolet range (300 to 800 nm) through an exposure process, and examples thereof include all of visual holograms such as in-line (Gabor) holograms, off-axis holograms, full-aperture transfer holograms, white light transmission holograms (“rainbow holograms”), Denisyuk holograms, off-axis reflection holograms, edge-lit holograms or holographic stereograms.

In the present disclosure, the alkyl group may be linear or branched, and the number of carbon atoms is not particularly limited, but is preferably 1 to 40. According to one embodiment, the alkyl group has 1 to 20 carbon atoms. According to another embodiment, the alkyl group has 1 to 10 carbon atoms. According to another embodiment, the alkyl group has 1 to 6 carbon atoms. Specific examples of the alkyl group include methyl, ethyl, propyl, n-propyl, isopropyl, butyl, n-butyl, isobutyl, tert-butyl, sec-butyl, 1-methyl-butyl, 1-ethyl-butyl, pentyl, n-pentyl, isopentyl, neopentyl, tert-pentyl, hexyl, n-hexyl, 1-methylpentyl, 2-methylpentyl, 4-methyl-2-pentyl, 3,3-dimethylbutyl, 2-ethylbutyl, heptyl, n-heptyl, 1-methylhexyl, cyclopentylmethyl, cycloheptylmethyl, octyl, n-octyl, tert-octyl, 1-methylheptyl, 2-ethylhexyl, 2-propylpentyl, n-nonyl, 2,2-dimethylheptyl, 1-ethyl-propyl, 1,1-dimethyl-propyl, isohexyl, 2-methylpentyl, 4-methylhexyl, 5-methylhexyl, and the like, but are not limited thereto.

In the present disclosure, the alkylene group is a divalent functional group derived from alkane, and may be linear, branched, or cyclic. Specific examples thereof include a methylene group, an ethylene group, a propylene group, an isobutylene group, a sec-butylene group, a tert-butylene group, a pentylene group, a hexylene group, and the like.

According to one embodiment of the present disclosure, provided is a photopolymer composition, including a polymer matrix or a precursor thereof including a (meth)acrylate-based (co)polymer having a silane-based functional group in a branched chain and a silane cross-linking agent; a photoreactive monomer; and a photoinitiator, wherein a ratio of functional groups represented by the following Chemical Formula 1 to siloxane functional groups is 90 mol % or more according to a solid state ²⁹Si NMR analysis of the silane cross-linking agent:

in Chemical Formula 1, R₁ to R₃ are each an organic functional group.

The present inventors have found through experiments that holograms formed from the photopolymer composition including a polymer matrix or a precursor thereof including a (meth)acrylate-based (co)polymer having a silane-based functional group in a branched chain and a silane cross-linking agent can exhibit remarkably improved refractive index modulation values and excellent durability against temperature and humidity even in thinner thickness ranges as compared with holograms previously known in the art, thereby completing the present invention.

When using a polymer matrix or a precursor thereof including a silane cross-linking agent in which a ratio of siloxane functional groups represented by the following Chemical Formula 1 to total siloxane functional groups is 90 mol % or more according to a solid state ²⁹Si NMR analysis and a (meth)acrylate-based (co)polymer having a silane-based functional group in a branched chain, cross-linking density is optimized at the time of manufacturing a coating film or a hologram from the photopolymer composition, and excellent durability against temperature and humidity compared with the existing matrix can be secured. In addition, by optimizing the cross-linking density, mobility between the photoreactive monomer having a high refractive index and other components having a low refractive index may be increased, thereby maximizing the refractive index modulation and improving recording characteristics.

Particularly, a cross-linked structure mediated by a siloxane bond may be easily introduced between a modified (meth)acrylate-based (co)polymer containing a silane-based functional group and a silane cross-linking agent containing a terminal silane-based functional group through a sol-gel reaction, and the siloxane bond can provide excellent durability against temperature and humidity.

In addition, a fluorine-based compound or a phosphate-based compound may further be added as a component of the polymer matrix or the precursor thereof. The fluorine-based compound or the phosphate-based compound has a lower refractive index than the photoreactive monomer, thereby lowering the refractive index of the polymer matrix and maximizing the refractive index modulation of the photopolymer composition.

Moreover, the phosphate-based compound serves as a plasticizer to lower the glass transition temperature of the polymer matrix, thereby increasing mobility of the photoreactive monomer and low refractive components, and contributing to the improvement of moldability of the photopolymer composition.

The polymer matrix or the precursor thereof including the (meth)acrylate-based (co)polymer having a silane-based functional group in a branched chain, and a silane cross-linking agent may serve as a support for the final product such as the photopolymer composition and a film produced therefrom. In the hologram formed from the photopolymer composition, the polymer matrix or the precursor thereof has a different refractive index to enhance the refractive index modulation.

As described above, the polymer matrix may include a (meth)acrylate-based (co)polymer in which a silane-based functional group is located in a branched chain, and a silane cross-linking agent. Accordingly, a precursor of the polymer matrix may include a monomer or an oligomer forming the polymer matrix. Specifically, it may include a (meth)acrylate-based (co)polymer having a silane-based functional group in a branched chain, a monomer thereof, or an oligomer of the monomer, and a silane cross-linking agent, a monomer thereof, or an oligomer of the monomer.

In the polymer matrix, the (meth)acrylate-based (co)polymer having a silane-based functional group in a branched chain and the silane cross-linking agent may be present as separate components, or in the form of a composite formed by reaction with each other.

A silane-based functional group may be located in a branched chain of the (meth)acrylate-based (co)polymer. The silane-based functional group may include a silane functional group or an alkoxysilane functional group, and preferably a trimethoxysilane group may be used as an alkoxysilane functional group.

The silane-based functional group may form a siloxane bond through a sol-gel reaction with the silane-based functional group contained in the silane cross-linking agent to cross-link the (meth)acrylate-based (co)polymer and the silane cross-linking agent.

A ratio of functional groups represented by the following Chemical Formula 1 to siloxane functional groups may be 90 mol % or more, or 95 mol % or more, preferably 99 mol % or more according to a solid state ²⁹Si NMR analysis of the silane cross-linking agent. The upper limit of the ratio is not particularly limited, but may be 100 mol % or 99.9 mol %.

The solid state ²⁹Si NMR analysis may be performed using a previously known instrument and method such as a Bruker AVANCE III 300 MHz instrument and a 7 mm MAS probe in a static condition. From the measured results, it is possible to confirm the above mol % range by calculating integral values.

The silane cross-linking agent in which the ratio of functional groups represented by the following Chemical Formula 1 to siloxane functional groups is 90 mol % or more according to the solid state ²⁹Si NMR analysis makes it possible to form more linear cross-linked structures inside the polymer matrix in the production of a hologram recording medium using the photopolymer composition of the embodiment. Therefore, the hologram recording medium may have improved recording characteristics, a relatively high refractive index modulation value (Δn), a high diffraction efficiency and a low laser loss (Moss). It is also possible to exhibit a characteristic in which the diffraction efficiency is not largely changed even when exposed to a temperature higher than room temperature and relatively high humidity.

More specifically, a ratio of siloxane functional groups represented by the following Chemical Formula 1 to siloxane functional groups represented by the following Chemical Formulae 1 to 4 may be 90 mol % or more, or 95 mol % or more, preferably 99 mol % or more according to a solid state ²⁹Si NMR analysis of the silane cross-linking agent.

In Chemical Formula 1, R₁ to R₃ are each an organic functional group.

In Chemical Formula 2, R₂₁ to R₂₄ are each an organic functional group.

In Chemical Formula 3, R₃₁ to R₃₅ are each an organic functional group.

In Chemical Formula 4, R₄₁ to R₄₆ are each an organic functional group.

In Chemical Formulae 1 to 4, when R₁ to R₃, R₂₁ to R₂₄, R₃₁ to R₃₅, and R₄₁ to R₄₆ are each organic functional groups, they do not greatly affect the bonding structure of Si in the solid state ²⁹Si NMR analysis, and therefore examples of the organic functional group are not particularly limited.

More specifically, R₁ to R₃ may each be hydrogen or a C1 to C20 alkyl group. In addition, R₂₁ to R₂₂ may each be hydrogen or a C1 to C20 alkyl group, and R₂₃ to R₂₄ may each be hydrogen, a C1 to C20 alkyl group, a hydroxy group or a C1 to C20 alkoxy group. In addition, R₃₁ may be hydrogen or a C1 to C20 alkyl group, and R₃₂ to R₃₅ may each be hydrogen, a C1 to C20 alkyl group, a hydroxy group or a C1 to C20 alkoxy group. In addition, R₄₁ to R₄₆ may each be hydrogen, a C1 to C20 alkyl group, a hydroxy group or a C1 to C20 alkoxy group.

The silane cross-linking agent may be a compound having an average of at least one silane-based functional group per molecule or a mixture thereof, and may be a compound containing at least one silane-based functional group. The silane-based functional group may include a silane functional group or an alkoxysilane functional group, and preferably a triethoxysilane group may be used as an alkoxysilane functional group. The silane-based functional group forms a siloxane bond through a sol-gel reaction with the silane-based functional group contained in the (meth)acrylate-based (co)polymer to cross-link the (meth)acrylate-based (co)polymer and the silane cross-linking agent.

Herein, the silane cross-linking agent may have an equivalent weight of the silane-based functional group of 200 g/equivalent to 1000 g/equivalent. Accordingly, cross-linking density between the (meth)acrylate-based (co)polymer and the silane cross-linking agent is optimized, so that excellent durability against temperature and humidity compared with the existing matrix can be secured. In addition, by optimizing the cross-linking density, mobility between the photoreactive monomer having a high refractive index and other components having a low refractive index can be increased, thereby maximizing the refractive index modulation and improving recording characteristics.

When the equivalent weight of the silane-based functional group contained in the silane cross-linking agent is excessively increased to 1000 g/equivalent or more, the diffraction grating interface after recording may collapse due to a decrease in the cross-linking density of the matrix. Further, loose cross-linking density and low glass transition temperature may cause the monomer and plasticizer components to be eluted onto the surface, thereby generating haze.

When the equivalent weight of the silane-based functional group contained in the silane cross-linking agent is excessively decreased to less than 200 g/equivalent, the cross-linking density increases too high, which hinders mobility of the monomer and plasticizer components, thereby deteriorating recording characteristics.

More specifically, the silane cross-linking agent may include a linear polyether main chain having a weight average molecular weight of 100 to 2000, 300 to 1000, or 300 to 700, and a silane-based functional group bound to the main chain as a terminal group or a branched chain.

The linear polyether main chain having a weight average molecular weight of 100 to 2000 may include a repeating unit represented by the following Chemical Formula 3.

—(R₈O)_(n)—R₈—   [Chemical Formula 3]

In Chemical Formula 3, R₈ is a C1 to C10 alkylene group, and n is an integer of 1 or more, 1 to 50, 5 to 20, or 8 to 10.

By introducing a flexible polyether polyol to the main chain of the silane cross-linking agent, the glass transition temperature and cross-linking density of the matrix may be controlled, resulting in improvement in mobility of the components.

Meanwhile, the bond between the silane-based functional group and the polyether main chain may be mediated by a urethane bond. Specifically, the silane-based functional group and the polyether main chain may form a mutual bond through a urethane bond. More specifically, a silicon atom contained in the silane-based functional group is bonded to a nitrogen atom of the urethane bond directly or via a C1 to C10 alkylene group, and a R₈ functional group contained in the polyether main chain may be directly bonded to an oxygen atom of the urethane bond.

The reason why the silane-based functional group and the polyether main chain are bonded through the urethane bond is that the silane cross-linking agent is produced by a reaction between an isocyanate compound containing a silane-based functional group and a linear polyether polyol compound having a weight average molecular weight of 100 to 2000.

More specifically, the isocyanate compound may include an aliphatic, cycloaliphatic, aromatic or aromatic-aliphatic mono-isocyanate, di-isocyanate, tri-isocyanate or poly-isocyanate; or oligo-isocyanate or poly-isocyanate of di-isocyanate or triisocyante having urethane, urea, carbodiimide, acylurea, isocyanurate, allophanate, biuret, oxadiazinetrione, uretdione or iminooxadiazinedione structures.

A specific example of the isocyanate compound containing the silane-based functional group may be 3-isocyanatopropyltriethoxysilane.

Further, the polyether polyol may be, for example, polyaddition products of styrene oxides, of ethylene oxide, of propylene oxide, of tetrahydrofuran, of butylene oxide, or of epichlorohydrin, mixed addition products thereof, grafting products thereof, polyether polyols obtained by condensation of polyhydric alcohols or mixtures thereof, and those obtained by alkoxylation of polyhydric alcohols, amines and amino alcohols.

Specific examples of the polyether polyol include poly(propylene oxide)s, poly(ethylene oxide)s and combinations thereof in the form of random or block copolymers, or poly(tetrahydrofuran)s and mixtures thereof having an OH functionality of 1.5 to 6 and a number average molecular weight of 200 to 18000 g/mol, preferably an OH functionality of 1.8 to 4.0 and a number average molecular weight of 600 to 8000 g/mol, particularly preferably an OH functionality of 1.9 to 3.1 and a number average molecular weight of 650 to 4500 g/mol.

When the silane-based functional group and the polyether main chain are bonded via a urethane bond, the silane cross-linking agent may be more easily synthesized.

The weight average molecular weight (measured by GPC) of the silane cross-linking agent may be 1000 to 5,000,000. The weight average molecular weight refers to a weight average molecular weight (unit: g/mol) using polystyrene calibration measured by a GPC method. In the process of measuring the weight average molecular weight using polystyrene calibration measured by a GPC method, a well-known analyzer, detector such as a refractive index detector, and analyzing column may be used. In addition, conventionally applied temperature conditions, solvents, and flow rates may be used. As a specific example of the measurement conditions, a temperature of 30° C., a chloroform solvent and a flow rate of 1 mL/min may be applied.

The (meth)acrylate-based (co)polymer may include a (meth)acrylate repeating unit in which a silane-based functional group is located in the branched chain, and a (meth)acrylate repeating unit.

An example of the (meth)acrylate repeating unit in which a silane-based functional group is located in the branched chain may be a repeating unit represented by the following Chemical Formula 1.

In Chemical Formula 1, R₁ to R₃ are each independently a C1 to C10 alkyl group, R₄ is hydrogen or a C1 to C10 alkyl group and R₅ is a C1 to C10 alkylene group.

Preferably, Chemical Formula 1 may be a repeating unit derived from 3-methacryloxypropyltrimethoxysilane (KBM-503) in which R₁ to R₃ are each independently a C1 methyl group, R₄ is a C1 methyl group and R₅ is a C3 propylene group, or a repeating unit derived from 3-acryloxypropyltrimethoxysilane (KBM-5103) in which R₁ to R₃ are each independently a C1 methyl group, R₄ is hydrogen and R₅ is a C3 propylene group.

In addition, an example of the (meth)acrylate repeating unit may be a repeating unit represented by the following Chemical Formula 2.

In Chemical Formula 2, R₆ is a C1 to C20 alkyl group, R₇ is hydrogen or a C1 to C10 alkyl group. Preferably, Chemical Formula 2 may be a repeating unit derived from butyl acrylate in which R₆ is a C4 butyl group and R₇ is hydrogen.

A molar ratio of the repeating unit of Chemical Formula 2 to the repeating unit of Chemical Formula 1 may be 0.5:1 to 14:1. When the molar ratio of the repeating unit of Chemical Formula 1 is excessively decreased, cross-linking density of the matrix becomes too low to serve as a support, and recording characteristics after recording may be deteriorated. When the molar ratio of the repeating unit of Chemical Formula 1 is excessively increased, cross-linking density of the matrix becomes too high and fluidity of the respective components may be decreased, resulting in a decrease in the refractive index modulation value.

The weight average molecular weight (measured by GPC) of the (meth)acrylate-based (co)polymer may be 100,000 to 5,000,000, or 300,000 to 900,000. The weight average molecular weight refers to a weight average molecular weight (unit: g/mol) using polystyrene calibration measured by a GPC method. In the process of measuring the weight average molecular weight using polystyrene calibration measured by a GPC method, a well-known analyzer, detector such as a refractive index detector, and analyzing column may be used. In addition, conventionally applied temperature conditions, solvents, and flow rates may be used. As a specific example of the measurement conditions, a temperature of 30° C., a chloroform solvent and a flow rate of 1 mL/min may be applied.

The (meth)acrylate-based (co)polymer may have an equivalent weight of the silane-based functional group of 300 g/equivalent to 2000 g/equivalent, 500 g/equivalent to 2000 g/equivalent, 550 g/equivalent to 1800 g/equivalent, 580 g/equivalent to 1600 g/equivalent, or 586 g/equivalent to 1562 g/equivalent.

The equivalent weight of the silane-based functional group corresponds to an equivalent weight per one silane-based functional group (g/equivalent) and is a value obtained by dividing the weight average molecular weight of the (meth)acrylate-based (co)polymer by the number of silane-based functional groups per molecule. The smaller the equivalent weight is, the higher the density of the functional group is. And the larger the equivalent weight, the smaller the density of the functional group.

Accordingly, cross-linking density between the (meth)acrylate-based (co)polymer and the silane cross-linking agent is optimized, and excellent durability against temperature and humidity compared with the existing matrix can be secured. In addition, by optimizing the cross-linking density, mobility between the photoreactive monomer having a high refractive index and other components having a low refractive index can be increased, thereby maximizing the refractive index modulation and improving recording characteristics.

When the equivalent weight of the silane-based functional group contained in the (meth)acrylate-based (co)polymer is excessively decreased to less than 300 g/equivalent, cross-linking density of the matrix increases too high, which hinders mobility of the components, thereby deteriorating recording characteristics. When the equivalent weight of the silane-based functional group contained in the (meth)acrylate-based (co)polymer is excessively increased to more than 2000 g/equivalent, cross-linking density becomes too low to serve as a support, so that the diffraction grating interface generated after recording may collapse, thereby reducing the refractive index modulation value over time.

Meanwhile, 10 parts by weight to 90 parts by weight, 20 parts by weight to 70 parts by weight, or 22 parts by weight to 65 parts by weight of the silane cross-linking agent may be included based on 100 parts by weight of the (meth)acrylate based (co)polymer.

In the reaction product, when the silane cross-linking agent content is excessively decreased with respect to 100 parts by weight of the (meth)acrylate-based (co)polymer, the curing rate of the matrix is significantly slowed to lose its function as a support, and the diffraction grating interface may collapse easily. When the silane cross-linking agent content is excessively increased, the curing rate of the matrix is accelerated, but an excessive increase in the reactive silane group content causes compatibility problems with other components, resulting in haze.

In addition, the modulus (storage modulus) of the reaction product may be 0.01 MPa to 5 MPa. For example, the storage modulus (G′) may be measured at a frequency of 1 Hz at room temperature (20° C. to 25° C.) using discovery hybrid rheometer (DHR) manufactured by TA Instruments.

Also, the glass transition temperature of the reaction product may be −40° C. to 10° C. For example, the glass transition temperature may be obtained by measuring a change in phase angle (loss modulus) of the film coated with the photopolymer composition using a dynamic mechanical analysis (DMA) apparatus at a temperature of −80° C. to 30° C. under the setting conditions of strain 0.1%, frequency 1 Hz, and heating rate 5° C./min.

The photoreactive monomer may include a polyfunctional (meth)acrylate monomer or a monofunctional (meth)acrylate monomer.

As described above, in a portion where the monomer is polymerized in the process of photopolymerization of the photopolymer composition and the polymer is present in relatively large amounts, the refractive index becomes high. In a portion where the polymer binder is present in relatively large amounts, the refractive index becomes relatively low, the refractive index modulation occurs, and a diffraction grating is generated by such refractive index modulation.

Specifically, an example of the photoreactive monomer may include (meth)acrylate-based α, β-unsaturated carboxylic acid derivatives, for example, (meth)acrylate, (meth)acrylamide, (meth)acrylonitrile, (meth)acrylic acid or the like, or a compound containing a vinyl group or a thiol group.

An example of the photoreactive monomer may include a polyfunctional (meth)acrylate monomer having a refractive index of 1.5 or more, 1.53 or more, or 1.5 to 1.7. The polyfunctional (meth)acrylate monomer having a refractive index of 1.5 or more, 1.53 or more, or 1.5 to 1.7 may include a halogen atom (bromine, iodine, etc.), sulfur (S), phosphorus (P), or an aromatic ring.

More specific examples of the polyfunctional (meth)acrylate monomer having a refractive index of 1.5 or more include bisphenol A modified diacrylate type, fluorene acrylate type (HR6022 etc. manufactured by Miwon Specialty Chemical Co., Ltd.), bisphenol fluorene epoxy acrylate type (HR6100, HR6060, HR6042, ect. manufactured by Miwon Specialty Chemical Co., Ltd.), halogenated epoxy acrylate type (HR1139, HR3362, etc. manufactured by Miwon Specialty Chemical Co., Ltd.), and the like.

Another example of the photoreactive monomer may include a monofunctional (meth)acrylate monomer. The monofunctional (meth)acrylate monomer may contain an ether bond and a fluorene functional group in the molecule. Specific examples thereof include phenoxybenzyl (meth)acrylate, o-phenylphenol ethylene oxide (meth)acrylate, benzyl (meth)acrylate, 2-(phenylthio)ethyl (meth)acrylate, biphenylmethyl (meth)acrylate, or the like.

Meanwhile, the photoreactive monomer may have a weight average molecular weight of 50 g/mol to 1000 g/mol, or 200 g/mol to 600 g/mol. The weight average molecular weight refers to a weight average molecular weight using polystyrene calibration measured by a GPC method.

Meanwhile, the photopolymer composition of the embodiment includes a photoinitiator. The photoinitiator is a compound which is activated by light or actinic radiation and initiates polymerization of a compound containing a photoreactive functional group such as the photoreactive monomer.

As the photoinitiator, commonly known photoinitiators can be used without particular limitation, but specific examples thereof include a photoradical polymerization initiator, a photocationic polymerization initiator and a photoanionic polymerization initiator.

Specific examples of the photoradical polymerization initiator include imidazole derivatives, bisimidazole derivatives, N-aryl glycine derivatives, organic azide compounds, titanocene, aluminate complex, organic peroxide, N-alkoxypyridinium salts, thioxanthone derivatives, amine derivatives, and the like. More specifically, examples of the photoradical polymerization initiator include 1,3-di(t-butyldioxycarbonyl)benzophenone, 3,3′,4,4″-tetrakis(t-butyldioxycarbonyl)benzophenone, 3-phenyl-5-isoxazolone, 2-mercapto benzimidazole, bis(2,4,5-triphenyl)imidazole, 2,2-dimethoxy-1,2-diphenylethane-1-one (product name: Irgacure 651/manufacturer: BASF), 1-hydroxy-cyclohexyl-phenyl-ketone (product name: Irgacure 184/manufacturer: BASF), 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1 (product name: Irgacure 369/manufacturer: BASF), bis(η5-2,4-cyclopentadiene-1-yl)-bis(2,6-difluoro-3-(1H-pyrrole-1-yl)-phenyl)titanium (product name: Irgacure/784 manufacturer: BASF), Ebecryl P-115(manufacturer: SK entis), and the like.

The photocationic polymerization initiator may include a diazonium salt, a sulfonium salt, or an iodonium saltm, and examples thereof include sulfonic acid esters, imidosulfonates, dialkyl-4-hydroxysulfonium salts, arylsulfonic acid-p-nitrobenzyl esters, silanol-aluminum complexes, (η6-benzene) (η5-cyclopentadienyl)iron (II), and the like. In addition, benzoin tosylate, 2,5-dinitrobenzyltosylate, N-tosylphthalic acid imide, and the like can be used. More specific examples of the photocationic polymerization initiator include commercially available products such as Cyracure UVI-6970, Cyracure UVI-6974 and Cyracure UVI-6990 (manufacturer: Dow Chemical Co. in USA), Irgacure 264 and Irgacure 250 (manufacturer: BASF) or CIT-1682 (manufacturer: Nippon Soda).

The photoanionic polymerization initiator may include a borate salt, and examples thereof include butyryl chlorine butyltriphenyl borate. More specific examples of the photoanionic polymerization initiator include commercially available products such as borate V (manufacturer: Spectra group).

In addition, the photopolymer composition of the embodiment may include a monomolecular (type I) initiator or a bimolecular (type II) initiator. The (type I) system for free radical photopolymerization may include, for example, an aromatic ketone compounds in combination with a tertiary amine, such as benzophenone, alkylbenzophenone, 4,4′-bis(dimethylamino)benzophenone (Michler's ketone), anthrone and halogenated benzophenone or a mixture of these types. The bimolecular (type II) initiator may include benzoin and derivatives thereof, benzyl ketal, acylphosphine oxide, for example, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, bisacylophosphine oxide, phenylglyoxyl ester, camphorquinone, alpha-aminoalkylphenone, alpha,alpha-dialkoxyacetophenone, 1-[4-(phenylthio)phenyl]octane-1,2-dione 2-(O-benzoyloxime), alpha-hydroxyalkylphenone, and the like.

The photopolymer composition may include 1 wt % to 80 wt % of the polymer matrix or the precursor thereof; 1 wt % to 80 wt % of the photoreactive monomer; and 0.1 wt % to 20 wt % of the photoinitiator. When the photopolymer composition further includes an organic solvent as described hereinafter, the content of the above-mentioned components is based on the sum of the above-mentioned components (the sum of the components excluding the organic solvent).

The photopolymer composition may further include a fluorine-based compound. The fluorine-based compound is stable with little reactivity and has a low refractive index. Therefore, the refractive index of the polymer matrix may be lowered when the fluorine-based compound is added into the photopolymer composition, thereby maximizing the refractive index modulation with the monomer.

The fluorine-based compound may include at least one functional group selected from the group consisting of an ether group, an ester group and an amide group, and at least two difluoromethylene groups. More specifically, the fluorine-based compound may have a structure represented by the following Chemical Formula 4 in which a functional group including an ether group is bonded to both terminal ends of a central functional group including a direct bond between two difluoromethylene groups or an ether bond.

In Chemical Formula 4, R₁₁ and R₁₂ are each independently a difluoromethylene group, R₁₃ and R₁₆ are each independently a methylene group, R₁₄ and R₁₅ are each independently a difluoromethylene group, and R₁₇ and R₁₈ are each independently a polyalkylene oxide, and m is an integer of 1 or more, 1 to 10, or 1 to 3.

Preferably in Chemical Formula 4, R₁₁ and R₁₂ are each independently a difluoromethylene group, R₁₃ and R₁₆ are each independently a methylene group, R₁₄ and R₁₅ are each independently a difluoromethylene group, and R₁₇ and R₁₈ are each independently a 2-methoxyethoxymethoxy group, and m is an integer of 2.

The fluorine-based compound may have a refractive index of less than 1.45, or 1.3 or more and less than 1.45. As described above, since the photoreactive monomer has a refractive index of 1.5 or more which is higher than that of the fluorine-based compound, the refractive index of the polymer matrix may be lowered, thereby maximizing the refractive index modulation with the monomer.

Specifically, the fluorine-based compound content may be 30 to 150 parts by weight, or 50 to 110 parts by weight based on 100 parts by weight of the photoreactive monomer, and the refractive index of the polymer matrix may be 1.46 to 1.53.

When the fluorine-based compound content is excessively decreased based on 100 parts by weight of the photoreactive monomer, the refractive index modulation value after recording may be lowered due to a lack of low refractive components. When the content is excessively increased, haze may be generated due to poor compatibility with other components or some fluorine-based compounds may be eluted to the surface of the coating layer.

The fluorine-based compound may have a weight average molecular weight (measured by GPC) of 300 or more, or 300 to 1000. A specific method of measuring the weight average molecular weight is as described above.

Meanwhile, the photopolymer composition may further include a photosensitizing dye. The photosensitizing dye serves as a photosensitizing pigment to sensitize the photoinitiator. More specifically, the photosensitizing dye may be stimulated by the light irradiated on the photopolymer composition and may also serve as an initiator to initiate polymerization of the monomer and the cross-linking monomer. The photopolymer composition may contain 0.01 wt % to 30 wt %, or 0.05 wt % to 20 wt % of the photosensitizing dye.

Examples of the photosensitizing dye are not particularly limited, and various compounds commonly known in the art can be used. Specific examples of the photosensitizing dye include sulfonium derivative of ceramidonine, new methylene blue, thioerythrosine triethylammonium, 6-acetylamino-2-methylceramidonin, eosin, erythrosine, rose bengal, thionine, basic yellow, Pinacyanol chloride, Rhodamine 6G, Gallocyanine, ethyl violet, Victoria blue R, Celestine blue, QuinaldineRed, Crystal Violet, Brilliant Green, Astrazon orange G, Darrow Red, Pyronin Y, Basic Red 29, pyrylium iodide, Safranin O, Cyanine, Methylene Blue, Azure A, or a combination of two or more thereof.

The photopolymer composition may further include an organic solvent. Examples of the organic solvent include ketones, alcohols, acetates, ethers, and a mixture of two or more thereof.

Specific examples of the organic solvent include ketones such as methyl ethyl ketone, methyl isobutyl ketone, acetylacetone or isobutyl ketone; alcohols such as methanol, ethanol, n-propanol, i-propanol, n-butanol, i-butanol or t-butanol; acetates such as ethyl acetate, i-propyl acetate, or polyethylene glycol monomethyl ether acetate; ethers such as tetrahydrofuran or propylene glycol monomethyl ether; or a mixture of two or more thereof.

The organic solvent may be added at the time of mixing the respective components contained in the photopolymer composition, or may be contained in the photopolymer composition by adding the respective components dispersed or mixed in an organic solvent. When the content of the organic solvent in the photopolymer composition is too low, flowability of the photopolymer composition may be lowered, resulting in the occurrence of defects such as striped patterns on the finally produced film. In addition, when too much organic solvent is added, the solid content is lowered, and coating and film formation are not sufficient, so that physical properties and surface characteristics of the film may be deteriorated and defects may occur during the drying and curing process. Thus, the photopolymer composition may include an organic solvent such that the total solid content concentration of the components contained is 1 wt % to 70 wt %, or 2 wt % to 50 wt %.

The photopolymer composition may further include other additives, catalysts, and the like. For example, the photopolymer composition may include a catalyst which is commonly known for promoting polymerization of the polymer matrix or the photoreactive monomer. Examples of the catalyst include tin octanoate, zinc octanoate, dibutyltin dilaurate, dimethylbis[(1-oxoneodecyl)oxy]stannane, dimethyltin dicarboxylate, zirconium bis(ethylhexanoate), zirconium acetylacetonate, p-toluenesulfonic acid, or tertiary amines such as 1,4-diazabicyclo[2.2.2]octane, diazabicyclononane, diazabicyclo undecane, 1,1,3,3-tetramethylguanidine, 1,3,4,6,7,8-hexahydro-1-methyl-2H-pyrimido(1,2-a)pyrimidine, and the like.

Examples of the other additives include a defoaming agent or a phosphate-based plasticizer, and the defoaming agent may be a silicone-based reactive additive, for example, Tego Rad 2500. Examples of the plasticizer include phosphate compounds such as tributyl phosphate, and the plasticizer may be added in a weight ratio of 1:5 to 5:1 together with the fluorine-based compound. The plasticizer may have a refractive index of less than 1.5 and a molecular weight of 700 or less.

The photopolymer composition can be used for hologram recording.

Meanwhile, according to another embodiment of the present disclosure, a hologram recording medium produced from the photopolymer composition may be provided.

As described above, when the photopolymer composition of one embodiment is used, it is possible to provide holograms capable of achieving a significantly improved refractive index modulation value and a high diffraction efficiency while having a thinner thickness, as compared with holograms previously known in the art.

The hologram recording medium can realize a refractive index modulation value (Δn) of 0.020 or more, 0.021 or more, 0.022 or more, 0.023 or more, 0.020 to 0.035, or 0.027 to 0.030 even at a thickness of 5 μm to 30 μm.

Further, the hologram recording medium may have a diffraction efficiency of 50% or more, 85% or more, or 85 to 99% at a thickness of 5 μm to 30 μm.

In the photopolymer composition of the one embodiment, the respective components contained therein are homogeneously mixed, dried and cured at a temperature of 20° C. or higher, and then predetermined exposure procedures are undertaken, thereby producing a hologram for optical application in the entire visible range and the near ultraviolet region (300 to 800 nm).

In the photopolymer composition of the one embodiment, the components for forming the polymer matrix or the precursor thereof may be first homogeneously mixed, and then the silane cross-linking agent may be mixed with the catalyst to prepare holograms.

In the photopolymer composition of one embodiment, a mixing device, a stirrer, a mixer, or the like which are commonly used in the art can be used for mixing the respective components contained therein without particular limitation. The temperature in the mixing process may be 0° C. to 100° C., preferably 10° C. to 80° C., particularly preferably 20° C. to 60° C.

Meanwhile, the components for forming the polymer matrix or the precursor thereof in the photopolymer composition of one embodiment are first homogenized and mixed. Subsequently, at the time of adding the silane cross-linking agent, the photopolymer composition may become a liquid formulation that is cured at a temperature of 20° C. or more.

The curing temperature may vary depending on the composition of the photopolymer and the curing is promoted, for example, by heating at a temperature of from 30° C. to 180° C.

At the time of curing, the photopolymer may be in state of being injected into or coated onto a predetermined substrate or mold.

Meanwhile, as the method of recording a visual hologram on a hologram recording medium produced from the photopolymer composition, generally known methods can be used without particular limitation. The method described in the holographic recording method of the embodiment described hereinafter can be adopted as an example.

According to another embodiment of the present disclosure, a holographic recording method may be provided, which includes selectively polymerizing photoreactive monomers contained in the photopolymer composition using a coherent laser.

As described above, through the process of mixing and curing the photopolymer composition, it is possible to produce a medium in which no visual hologram is recorded, and a visual hologram can be recorded on the medium through a predetermined exposure process.

A visual hologram can be recorded on the media provided through the process of mixing and curing the photopolymer composition, using known devices and methods under commonly known conditions.

According to another embodiment of the present disclosure, an optical element including the hologram recording medium may be provided.

Specific examples of the optical element include optical lenses, mirrors, deflecting mirrors, filters, diffusing screens, diffraction elements, light guides, waveguides, holographic optical elements having projection screen and/or mask functions, medium of optical memory system and light diffusion plate, optical wavelength multiplexers, reflection type, transmission type color filters, and the like.

An example of the optical element including the hologram recording medium may include a hologram display device.

The hologram display device includes a light source unit, an input unit, an optical system, and a display unit. The light source unit is a part that irradiates a laser beam used for providing, recording, and reproducing three-dimensional image information of an object in the input unit and the display unit. Further, the input unit is a part that previously inputs three-dimensional image information of an object to be recorded on the display unit, and for example, three-dimensional information of an object such as the intensity and phase of light for each space can be input into an electrically addressed liquid crystal SLM, wherein an input beam may be used. The optical system may include a mirror, a polarizer, a beam splitter, a beam shutter, a lens, and the like. The optical system can be distributed into an input beam for sending a laser beam emitted from the light source unit to the input unit, a recording beam for sending the laser beam to the display unit, a reference beam, an erasing beam, a reading beam, and the like.

The display unit can receive three-dimensional image information of an object from an input unit, record it on a hologram plate composed of an optically addressed SLM, and reproduce the three-dimensional image of the object. Herein, the three-dimensional image information of the object can be recorded via interference of the input beam and the reference beam. The three-dimensional image information of the object recorded on the hologram plate can be reproduced into a three-dimensional image by the diffraction pattern generated by the reading beam. The erasing beam can be used to quickly remove the formed diffraction pattern. Meanwhile, the hologram plate can be moved between a position at which a three-dimensional image is input and a position at which a three-dimensional image is reproduced.

Advantageous Effects

According to the present disclosure, provided are a photopolymer composition which can more effectively and easily provide a photopolymer layer having a high refractive index modulation value even with a thin thickness, a hologram recording medium having a high refractive index modulation value even with a thin thickness, an optical element and a holographic recording method.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, the present invention will be explained in detail with reference to the following examples. However, these examples are only to illustrate the invention, and the scope of the invention is not limited thereto.

Preparation Examples Preparation Example 1: Preparation of (Meth)Acrylate-Based (Co)Polymer Having a Silane-Based Functional Group in a Branched Chain

154 g of butyl acrylate and 46 g of KBM-503 (3-methacryloxypropyltrimethoxysilane) were placed in a 2 L jacket reactor and diluted with 800 g of ethyl acetate. The reaction temperature was set at about 60 to 70 t and stirring was continued for about 30 minutes to 1 hour. 0.02 g of n-dodecyl mercaptan was further added thereto, and stirring was further continued for about 30 minutes. Thereafter, 0.06 g of AIBN as a polymerization initiator was added thereto, and polymerization was continued for 4 hours or more at the reaction temperature until the residual acrylate content became less than 1% to obtain a (meth)acrylate-based (co)polymer having a silane-based functional group in a branched chain (the weight average molecular weight was about 500,000 to 600,000, and the equivalent weight of Si—(OR)₃ was 1019 g/equivalent).

Preparation Example 2: Preparation of (Meth)Acrylate-Based (Co)Polymer Having a Silane-Based Functional Group in a Branched Chain

180 g of butyl acrylate and 120 g of KBM-503 (3-methacryloxypropyltrimethoxysilane) were placed in a 2 L jacket reactor and diluted with 700 g of ethyl acetate. The reaction temperature was set at about 60 to 70 t and stirring was continued for about 30 minutes to 1 hour. 0.03 g of n-dodecyl mercaptan was further added thereto, and stirring was further continued for about 30 minutes. Thereafter, 0.09 g of AIBN as a polymerization initiator was added thereto, and polymerization was continued for 4 hours or more at the reaction temperature until the residual acrylate content became less than 1% to obtain a (meth)acrylate-based (co)polymer having a silane-based functional group in a branched chain (the weight average molecular weight was about 500,000 to 600,000, and the equivalent weight of Si—(OR)₃ was 586 g/equivalent).

Preparation Example 3: Preparation of (Meth)Acrylate-Based (Co)Polymer Having a Silane-Based Functional Group in a Branched Chain

255 g of butyl acrylate and 45 g of KBM-503 (3-methacryloxypropyltrimethoxysilane) were placed in a 2 L jacket reactor and diluted with 700 g of ethyl acetate. The reaction temperature was set at about 60 to 70 t and stirring was continued for about 30 minutes to 1 hour. 0.03 g of n-dodecyl mercaptan was further added thereto, and stirring was further continued for about 30 minutes. Thereafter, 0.09 g of AIBN as a polymerization initiator was added thereto, and polymerization was continued for 4 hours or more at the reaction temperature until the residual acrylate content became less than 1% to obtain a (meth)acrylate-based (co)polymer having a silane-based functional group in a branched chain (the weight average molecular weight was about 500,000 to 600,000, and the equivalent weight of Si—(OR)₃ was 1562 g/equivalent).

Preparation Example 4: Preparation of the Above-Mentioned Silane Cross-Linking Agent

19.79 g of KBE-9007 (3-isocyanatopropyltriethoxysilane), 12.80 g of PEG-400 and 0.57 g of DBTDL were placed in a 1000 ml flask, and diluted with 300 g of tetrahydrofuran. After stirring at room temperature until all of the reactants were confirmed to be consumed by TLC, the reaction solvent was completely removed under reduced pressure.

28 g of a liquid product having a purity of 95% or more was separated at a yield of 91% through column chromatography under a developing solution (dichloromethane: methyl alcohol=30:1) to obtain the above-mentioned silane cross-linking agent.

Herein, the solid state ²⁹Si NMR analysis of the silane cross-linking agent was performed using a Bruker AVANCE III 300 MHz instrument and a 7 mm MAS probe in a static condition. By calculating integral values, it was confirmed that a ratio of functional groups represented by the following Chemical Formula 1 to functional groups represented by the following Chemical Formulae 1 to 4 of the siloxane functional groups was about 95 mol % (the description of the functional groups represented by the following Chemical formulae 1 to 4 is as described above).

Preparation Example 5: Preparation of Non-Reactive Low Refractive Material

20.51 g of 2,2′-((oxybis(1,1,2,2-tetrafluoroethane-2,1-diyl))bis(oxy))bis(2,2-difluoroethan-1-ol was placed in a 1000 ml flask, dissolved in 500 g of tetrahydrofuran, and 4.40 g of sodium hydride (60% dispersion in mineral oil) was gently added several times while stirring at 0° C. After stirring at 0° C. for 20 minutes, 12.50 ml of 2-methoxyethoxymethyl chloride was slowly dropped. When all of the reactants were confirmed to be consumed by ¹H NMR, the reaction solvent was completely removed under reduced pressure. The organic layer was collected by extracting three times with 300 g of dichloromethane. Thereafter, it was filtered with magnesium sulfate, and all dichloromethane was removed under reduced pressure to obtain 29 g of a liquid product having a purity of 95% or more at a yield of 98%.

Examples and Comparative Examples: Preparation of Photopolymer Composition

As shown in Table 1 or Table 2 below, the silane polymer obtained in Preparation Examples 1 to 3, the photoreactive monomer (high refractive acrylate, refractive index of 1.600, HR6022, manufactured by Miwon), the non-reactive low refractive material of Preparation Example 5, tributyl phosphate (TBP, molecular weight of 266.31, refractive index of 1.424, manufactured by Sigma-Aldrichch), safranin O (dye, manufactured by Sigma-Aldrich), Ebecryl P-115 (manufactured by SK entis), Borate V (manufactured by Spectra group), Irgacure 250 (manufactured by BASF), the silicone-based reactive additive (Tego Rad 2500), and methyl isobutyl ketone (MIBK) were mixed with light blocked, and stirred with a paste mixer for about 10 minutes to obtain a transparent coating solution.

The above-described silane cross-linking agent of Preparation Example 4 or a silane cross-linking agent with a network structure (Sibond H-5) was added to the coating solution, and further stirred for 5 to 10 minutes. Thereafter, 0.02 g of DBTDL as a catalyst was added to the coating solution, and stirred for about 1 minute. Then, the coating solution was coated on a TAC substrate (80 μm) using a meyer bar to a thickness of 6 μm, and dried at 40° C. for 1 hour.

Thereafter, the sample was allowed to stand for 24 hours or more in a dark room under constant temperature and humidity conditions of about 25° C. and 50 RH %.

Experimental Examples: Holographic Recording

(1) The photopolymer-coated surfaces prepared in each of Examples and Comparative Examples were laminated on a slide glass, and fixed so that a laser first passed through the glass surface at the time of recording.

(2) Measurement of diffraction efficiency (η)

A holographic recording was done via interference of two interference lights (reference light and object light), and a transmission-type recording was done so that the two beams were incident on the same side of the sample. The diffraction efficiencies change with the incident angle of the two beams, and become non-slanted when the incident angles of the two beams are the same. In the non-slanted recording, the diffraction grating is generated perpendicularly to the film because the incident angles of the two beams are equal to a normal line.

The recording (2θ=45°) was done in a transmission-type non-slanted manner using a laser with a wavelength of 532 nm, and the diffraction efficiency (n) was calculated according to the following Equation 1.

$\begin{matrix} {\eta = \frac{P_{D}}{P_{D} + P_{T}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack \end{matrix}$

In Equation 1, η is a diffraction efficiency, P_(D) is an output amount (mW/cm²) of the diffracted beam of a sample after recording, and P_(T) is an output amount (mW/cm²) of the transmitted beam of the recorded sample.

(3) Measurement of refractive index modulation value (n)

The lossless dielectric grating of the transmission-type hologram can calculate the refractive index modulation value (Δn) from the following Equation 2.

$\begin{matrix} {{\eta ({DE})} = {{\sin^{2}\left( \sqrt{v^{2}} \right)} = {\sin^{2}\left( \frac{\pi \; \Delta \; {nd}}{\lambda \; \cos \; \theta} \right)}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack \end{matrix}$

In Equation 2, d is a thickness of the photopolymer layer, Δn is a refractive index modulation value, η(DE) is a diffraction efficiency, and λ is a recording wavelength.

(4) Measurement of laser loss (I_(loss))

The laser loss (I_(loss)) can be calculated from the following Equation 3.

I _(loss)=1−{(P _(D) +P _(T))/I _(O)}  [Equation 3]

In Equation 3, P_(D) is an output amount (mW/cm²) of the diffracted beam of a sample after recording, P_(T) is an output amount (mW/cm²) of the transmitted beam of the recorded sample, and I₀ is an intensity of the recording light.

TABLE 1 The photopolymer compositions (unit: g) of Examples and measurement results of Experimental Examples of the holographic recording medium prepared therefrom Example 1 Example 2 Example 3 Example 4 (Meth)acrylate- Prep. Example 1 23.1 23.1 based copolymer Prep. Example 2 19.3 Prep. Example 3 25.4 Linear silane cross- Prep. Example 4 8.4 8.4 12.3 6.1 linking agent Silane cross-linking Sibond H-5 agent with a network structure Photoreactive HR6022 31.5 31.5 31.5 31.5 monomer Dye safranin O 0.1 0.1 0.1 0.1 Amine Ebecryl P-115 1.7 1.7 1.7 1.7 Borate salt Borate V 0.3 0.3 0.3 0.3 Onium salt Irgacure 250 0.1 0.1 0.1 0.1 Non-reactive Tributyl phosphate 0 17.2 17.2 17.2 plasticizer (TBP) Non-reactive low Prep. Example 5 34.4 17.2 17.2 17.2 refractive material (P3) Catalyst DBTDL(dibutyltin 0.02 0.02 0.02 0.02 dilaurate) Additive Tego Rad 2500 0.3 0.3 0.3 0.3 Solvent MIBK 300 300 300 300 Coating thickness (unit: μm) 6 6 6 6 I_(loss)(%) 25 19 21 20 Δn 0.023 0.027 0.030 0.022

* The silane cross-linking agent with a network structure: Silbond H-5 (27% in Ethanol)/a ratio of functional groups represented by the Chemical Formula 4 is about 16 mol % according to a solid state ²⁹Si NMR analysis (functional groups of the Chemical Formula 2:32%, functional groups of the Chemical Formula 3:52%)

** The non-reactive plasticizer: Tributyl phosphate (molecular weight of 266.31, refractive index of 1.424, manufactured by Sigma-Aldrichch)

TABLE 2 The photopolymer compositions of Comparative Examples and measurement results of Experimental Examples of the holographic recording medium prepared therefrom Comp. Ex. 1 Comp. Ex. 2 Comp. Ex. 3 (Meth)acrylate-based Prep. Ex. 1 31.4 copolymer Prep. Ex. 2 31.4 Prep. Ex. 3 31.4 Silane cross-linking agent Sibond H-5 0.1 0.1 0.1 with a network structure Photoreactive monomer HR6022 31.4 31.4 31.4 Dye safranin O 0.1 0.1 0.1 Amine Ebecryl P-115 1.7 1.7 1.7 Borate salt Borate V 0.3 0.3 0.3 Onium salt Irgacure 250 0.1 0.1 0.1 Non-reactive plasticizer Tributyl phosphate 17.2 17.2 17.2 (TBP) Non-reactive low refractive Prep. Ex. 5 17.2 17.2 17.2 material (P3) Catalyst DBTDL(dibutyltin 0.02 0.02 0.02 dilaurate) Additive Tego Rad 2500 0.3 0.3 0.3 Solvent MIBK 300 300 300 Coating thickness (unit: μm) 6 6 6 I_(loss)(%) 25 28 27 Δn 0.018 0.015 0.019

As shown in Tables 1 and 2 above, it was confirmed that the photopolymer compositions of Examples using the polymer matrix in which the degree of cross-linking was controlled by using the silane cross-linking agent having functional groups of Chemical Formula 1 in a ratio of 90 mol % or more (Preparation Example 4) together with the polymers prepared in Preparation Examples 1 to 3 have a refractive index modulation value (Δn) of 0.020 or more.

On the contrary, it was confirmed that the photopolymer coating films prepared from the compositions of Comparative Examples have a relatively low diffraction efficiency and a relatively high laser loss (Moss) than that of Examples. 

1. A photopolymer composition, comprising a polymer matrix or a precursor thereof including a (meth)acrylate-based (co)polymer having a silane-based functional group in a branched chain and a silane cross-linking agent; a photoreactive monomer; and a photoinitiator, wherein the silane cross-linking agent contains 90 mol % or more of siloxane functional groups represented by the following Chemical Formula 1 based on total siloxane functional groups therein according to a solid state ²⁹Si NMR analysis of the silane cross-linking agent:

in Chemical Formula 1, R₁ to R₃ are each an organic functional group.
 2. The photopolymer composition of claim 1, wherein the total siloxane functional groups includes siloxane functional groups represented by Chemical Formulae 1 to 4:

in Chemical Formula 1, R₁ to R₃ are each an organic functional group,

in Chemical Formula 2, R₂₁ to R₂₄ are each an organic functional group,

in Chemical Formula 3, R₃₁ to R₃₅ are each an organic functional group,

in Chemical Formula 4, R₄₁ to R₄₆ are each an organic functional group.
 3. The photopolymer composition of claim 1, wherein the (meth)acrylate-based (co)polymer has an equivalent weight of the silane-based functional group of 300 g/equivalent to 2000 g/equivalent.
 4. The photopolymer composition of claim 1, wherein the silane cross-linking agent is contained in an amount of 10 parts by weight to 90 parts by weight based on 100 parts by weight of the (meth)acrylate-based (co)polymer.
 5. The photopolymer composition of claim 1, wherein the silane cross-linking agent comprises a linear polyether main chain having a weight average molecular weight of 100 to 2000 and a silane-based functional group bound to the main chain as a terminal group or a branched chain.
 6. The photopolymer composition of claim 5, wherein the silane-based functional group is bound to the polyether main chain by a urethane bond.
 7. The photopolymer composition of claim 1, wherein the silane cross-linking agent has an equivalent weight of the silane-based functional group of 200 g/equivalent to 1000 g/equivalent.
 8. The photopolymer composition of claim 1, wherein the photoreactive monomer comprises a polyfunctional (meth)acrylate monomer or a monofunctional (meth)acrylate monomer.
 9. The photopolymer composition of claim 1, wherein the photoreactive monomer has a refractive index of 1.5 or more.
 10. The photopolymer composition of claim 1, comprising: 1 wt % to 80 wt % of the polymer matrix or the precursor thereof; 1 wt % to 80 wt % of the photoreactive monomer; and 0.1 wt % to 20 wt % of the photoinitiator.
 11. The photopolymer composition of claim 1, further comprising a fluorine-based compound.
 12. The photopolymer composition of claim 11, wherein the fluorine-based compound comprises at least one functional group selected from the group consisting of an ether group, an ester group and an amide group, and at least two difluoromethylene groups.
 13. The photopolymer composition of claim 11, wherein the fluorine-based compound has a refractive index of less than 1.45.
 14. The photopolymer composition of claim 1, wherein the polymer matrix has a refractive index of 1.46 to 1.53.
 15. The photopolymer composition of claim 1, further comprising a photosensitizing dye, or other additives.
 16. A hologram recording medium produced from the photopolymer composition of claim
 1. 17. An optical element comprising the hologram recording medium of claim
 16. 18. A holographic recording method comprising selectively polymerizing photoreactive monomers contained in the photopolymer composition of claim 1 using a coherent laser. 